ABUJA, March 13 (Reuters) - A Nigerian group that sparked a global campaign for the safe return of schoolgirls kidnapped in 2014 said on Tuesday it would sue the government if it failed to provide answers over a similar mass abduction last month.

Islamist militant group Boko Haram took 110 schoolgirls from the northeastern town of Dapchi on Feb. 19. and, under a previous administration, it kidnapped 276 girls from the town of Chibok in 2014.

"The magnitude of incompetence and carelessness of our government enabled the repeat of the worst abduction tragedy," said campaign group Bring Back Our Girls (BBOG).

The campaign group was formed in the wake of the Chibok abduction and it submitted questions about the Dapchi kidnapping to President Muhammadu Buhari's government.

BBOG challenged the government over how Boko Haram could have staged the attack, given that the administration and the military say the insurgent group is defeated.

They said the Dapchi school was unprotected despite a Safe Schools Initiative set up in 2014 and the military had earlier withdrawn from the town at an unspecified time. The school is located in northeast Nigeria, the heartland of the insurgency.

The campaign group also asked why the government said in its initial response to the Dapchi incident that there was no evidence of abduction and why parents were later told more than 70 girls had been returned, information that proved false.

"Our questions are premised on the troubling sparseness of information ... by the federal government," BBOG said in a statement. A Nigerian presidency spokesman declined to comment.

Boko Haram has killed 20,000 people since 2009 but it no longer holds the large tracts of territory that it did in late 2014. Even so, it conducts raids in remote areas, making it hard for security forces to defend people in areas largely cut off from the rest of the country.

If the administration fails to respond within seven days BBOG will sue the government for gross negligence that led to the Chibok abductions, the group's founder, Obi Ezekwesili, told reporters in the capital Abuja.

Similar criticisms were levelled by Buhari's then-opposition party at the administration of former president Goodluck Jonathan after the Chibok kidnapping.

Some Chibok girls have been freed after what security sources say were ransom payments but around 100 are still being held. (Additional reporting by Felix Onuah Writing by Paul Carsten Editing by Matthew Mpoke Bigg)